JP5054463B2 - Josephson junction element, formation method thereof, and superconducting junction circuit - Google Patents

Description

  The present invention relates to a Josephson junction element using an oxide superconductor, a method for forming the same, and a superconducting junction circuit using the Josephson junction element.

Since the discovery of a copper oxide superconductor of YBa ₂ Cu ₃ O _y (YBCO) with a critical temperature of 92K in 1987, research on oxide superconductors has been actively conducted. A Josephson junction element using an oxide superconductor has a high IcRn product and a high junction superconducting transition temperature. The IcRn product is a product of the maximum superconducting current value (critical current value) Ic that can flow in the superconducting junction at a certain temperature and the resistance value Rn when the superconducting state is broken and becomes a normal conducting state. Is an index representing the magnitude of the signal and the sensitivity at the time of switching. Therefore, using such a Josephson junction element, high-speed operation of the superconducting junction circuit at a temperature of 40 K or higher and high sensitivity of the sensor can be achieved.

  In order to realize a device using an oxide superconductor, a fabrication technique of a Josephson junction element using the Josephson effect is indispensable. As a Josephson junction element using an oxide superconductor, various structures such as a bicrystal type, a micro bridge type, a step edge type, and a lamp edge type have been proposed.

  Bicrystal type bonding is easy because a superconducting film is deposited on a substrate with different crystal orientations bonded together, and a grain boundary that acts as a barrier layer is formed in the superconducting film on the substrate boundary. It is possible to produce. Devices such as SQUID elements using this bonding have already reached the stage of commercialization. However, since the bicrystal type can produce a junction only at the bonding boundary of the substrates, it is not suitable for high-function device applications that require a complicated structure.

On the other hand, a stacked Josephson junction element, or an upper electrode layer 104 stacked on a substrate 101 with a lower electrode layer 102 made of an oxide superconductor and an insulating film 103 shown in FIG. In the ramp-edge type Josephson junction element having a junction formed on the slope contacting with the barrier layer 102a, there is no limitation on the junction formation location, which is very advantageous for large-scale integration of superconducting junction circuits. Many barrier layers having these structures use non-superconducting materials such as PrBa ₂ Cu ₃ Oy, SrTiO ₃ , PrGaO ₃ , and CeO ₂ , and many use an interface-modified junction (Interface-Engineered Junction). It has been reported. In particular, as shown in FIG. 1B, in the interface-modified bonding, the lower electrode layer 102 and the insulating film 103 are stacked on the substrate 101, and the slope formed on the lower electrode layer is made amorphous by the impact of Ar ions. Thus, a thin barrier layer having a thickness of several nanometers is formed, and the upper electrode layer covering the barrier layer is formed. Therefore, the Josephson junction element of the interface modification type junction has a characteristic that suppresses variation in junction characteristics and has a relatively high IcRn product as compared with a junction using a non-superconducting material.

In the case of a Josephson junction element with an interface-modified junction, an experiment with 100 junctions at a temperature of 4.2 K and an IcRn product of 1.5 to 3 mV and an Ic 1σ (σ: standard deviation) of about 8% has been conducted so far. Results (see Non-Patent Document 1) and experimental results with an IcRn product of 1.5 mV and an Ic 1σ (σ: standard deviation) of about 7.9% at a temperature of 4.2 K (Non-Patent Document 1) 2) has been reported.
T. Satoh et al., "High-Temperature Superconducting Edge-Type Josephson Junctions with Modified Interfaces", IEEE Trans. Appl. Supercond. Vol. 9, No. 2, June 1999, pp.3141-3144 Y. Soutome et al., "HTS Surface-Modified Junctions with Integrated Ground-Planes for SFQ Circuits", IEICE Trans. Electron. Vol.E85-C, No.3, March 2002, pp.759-763 JG Wen et al., "Atomic structure and composition of the barrier in the modified interface high-Tc Josephson junction studied by transmission electron microscopy", Appl. Phys. Lett., Vol. 75, No. 16, 18 October 1999, pp .2470-2472 S. Adachi et al., "Structure and Formation Mechanism of Interface-Modified Layer in Ramp-Edge Josephson Junctions With La-Doped 123-Type Superconducting Electrodes", IEEE Trans. Appl. Supercond., Vol.13, No.2, June 2003, pp.877-880

  When considering application to Josephson junction devices, a high-speed operation at high temperature and a high-sensitivity Josephson junction element are essential from the viewpoint of system cost reduction. For that purpose, a Josephson junction element having a larger IcRn product is desired, but the above-mentioned Non-Patent Documents 1 and 2 do not provide a sufficient IcRn product.

  Therefore, an object of the present invention is to provide a Josephson junction element having an improved IcRn product, a method for forming the same, and a superconducting junction circuit using the Josephson junction element.

According to one aspect of the present invention, there is provided a Josephson junction element in which a first superconducting electrode layer, a barrier layer, and a second superconducting electrode layer are stacked in this order, wherein the first and second The superconducting electrode layer is made of an oxide superconducting material mainly composed of (RE) ₁ (AE) ₂ Cu ₃ O _y , and the element RE includes Y, La, Pr, Nd, Sm, Eu, Gd, Dy, and Ho. , Er, Tm, Yb, and Lu, and the element AE is at least one of the group consisting of Ba, Sr, and Ca, and the barrier layer includes the element RE, the element It is made of a material containing AE, Cu, and oxygen, and among the cations in the material, the Cu content is set to a range of 35 atomic% to 55 atomic%, and the elemental RE content is set to a range of 12 atomic% to 30 atomic%. And the first and second superelectrics Josephson device is provided, wherein the composition an electrode layer are different.

  According to the present invention, since the barrier layer is a deposited film of an oxide material, it is possible to avoid the problem that the Cu content of the barrier layer becomes extremely small as in the conventional interface-modified bonding, and a high-quality superconducting junction can be obtained. Can be formed. As a result, the IcRn product of the Josephson junction element can be improved. Furthermore, variations in the critical current Ic can be suppressed. In addition, there is an effect that the critical temperature of the superconducting junction becomes higher than that of the conventional interface reforming type. As a result, the cooling cost of the Josephson junction element can be reduced. The compositions of the first and second superconducting electrode layers are preferably detected by the same analysis method as that for the barrier layer and compared with, for example, about 10 nm from the interface with the barrier layer. This is the same in the present specification and claims.

According to another aspect of the present invention, the substrate, the first superconducting electrode layer having one inclined end surface on the substrate, and the inclined surface on the first superconducting electrode layer are continuous. An insulating film having an inclined surface formed on an end surface; a barrier layer deposited on a surface of the inclined surface of the first superconducting electrode layer; and a second superconducting electrode layer covering the barrier layer, The second superconducting electrode layer is made of an oxide superconducting material whose main component is (RE) ₁ (AE) ₂ Cu ₃ O _y , and the element RE includes Y, La, Pr, Nd, Sm, Eu, Gd, At least one selected from the group consisting of Dy, Ho, Er, Tm, Yb, and Lu, and the element AE is at least one selected from the group consisting of Ba, Sr, and Ca; Made of material containing RE, element AE, Cu, and oxygen, the material Among the cations therein, the Cu content is set in the range of 35 atomic% to 55 atomic%, the element RE content is set in the range of 12 atomic% to 30 atomic%, and the first and second superconducting electrode layers There is provided a Josephson junction device characterized by a different composition.

  According to the present invention, a ramp-edge type Josephson junction element having the same effect as the above-described invention is provided.

According to another aspect of the present invention, a substrate, a first superconducting electrode layer formed on the substrate, and an insulating film having a through hole along the thickness direction on the first superconducting electrode layer A barrier layer deposited on the surface of the first superconducting electrode layer in the through hole, and a second superconducting electrode layer filling the through hole and covering the barrier layer and the insulating film. The second superconducting electrode layer is made of an oxide superconducting material mainly composed of (RE) ₁ (AE) ₂ Cu ₃ O _y , and the element RE is Y, La, Pr, Nd, Sm, Eu, Gd. , Dy, Ho, Er, Tm, Yb, and Lu, and the element AE is at least one of the group consisting of Ba, Sr, and Ca, and the barrier layer includes: From material containing element RE, element AE, Cu, and oxygen Among the cations in the material, the Cu content is set to a range of 35 atomic% to 55 atomic%, the element RE content is set to a range of 12 atomic% to 30 atomic%, and the first and second A Josephson junction device having a composition different from that of the superconducting electrode layer is provided.

  According to the present invention, there is provided a multilayer Josephson junction element having the same effect as the above-described invention.

  According to another aspect of the present invention, a step of forming a first superconducting electrode layer of an oxide superconducting material on a substrate, a step of forming an insulating film covering the first superconducting electrode layer, and the insulating film Removing a part of the first superconducting electrode layer to expose the first superconducting electrode layer; depositing a barrier layer on the exposed surface of the first superconducting electrode layer using an oxide material; and Forming a second superconducting electrode layer of the covering oxide superconducting material, wherein the oxide material is made of a material containing the element RE, the elements AE, Cu, and oxygen, and the element RE is composed of Y, La, At least one selected from the group consisting of Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and the element AE is at least one selected from the group consisting of Ba, Sr, and Ca. Josepho characterized as a seed Method of forming a junction element is provided.

  According to the present invention, by depositing and forming the barrier layer, the film quality of the barrier layer is improved and the IcRn product is improved as compared with the conventional interface-modified barrier layer. In particular, even if the Cu content of the barrier layer is a high Cu content such that current leakage occurs between the first superconducting electrode layer and the second superconducting electrode layer in the conventional interface reforming type, the crystal Since it is possible to avoid damage inside, current leakage can be suppressed.

  ADVANTAGE OF THE INVENTION According to this invention, the Josephson junction element which improved IcRn product, its formation method, and the superconducting junction circuit using a Josephson junction element can be provided.

As a result of diligent research, the inventors of the present application have determined that the superconducting material of the first and second superconducting electrode layers is a superconducting oxide mainly composed of (RE) ₁ (AE) ₂ Cu ₃ O _y in the Josephson junction element. Excellent junction characteristics such as IcRn product by adopting a deposited film made of an oxide material of a predetermined composition range as a material and as a barrier layer of a superconducting junction between the first superconducting electrode layer and the second superconducting electrode layer It was learned that Here, the element RE is at least one selected from the group consisting of Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and the element AE is Ba, Sr. And at least one selected from the group consisting of Ca.

  The barrier layer of the Josephson junction device of the present invention is a deposited film containing the element RE, the element AE, Cu, and oxygen, and among the cations in the oxide material constituting the barrier layer, the Cu content is 35. Atomic% to 55 atomic%, and the elemental RE content is set in the range of 12 atomic% to 30 atomic%, and the composition is different from that of the first and second superconducting electrode layers. The inventors of the present application have found that the use of such a barrier layer improves the IcRn product by 30% or more as compared with a Josephson junction element having a conventional interface-modified junction. This effect is presumed as follows.

When an oxide superconducting material is used for a Josephson junction element, it is theoretically said that the IcRn product is 10 mV or more. However, in the Josephson junction element of the interface modification type junction, a junction having a very large IcRn product is not actually obtained. For example, a barrier layer formed by bombarding Ar ions with an oxide superconducting material of YBa ₂ Cu ₃ O _y (Y: Ba: Cu = 17: 33: 50) is, for example, Y: Ba: Cu = 43: The composition ratio is 30:27 (Non-Patent Document 3) and (Y + Yb) :( Ba + La): Cu = 20: (38 + 10): 32 (Non-Patent Document 4), and the Cu content is significantly reduced. It has been reported that In addition, the IcRn product of both the fabricated junctions was about 2 mV, which was not sufficient. The barrier layer of these interface modified junctions is said to be a crystalline phase having a perovskite structure. The perovskite structure is represented by the chemical formula ABO ₃ , and A ions and B ions are surrounded by 12 and 6 oxygen atoms, respectively. A cation with a relatively large ion radius (cation) occupies the position of the A ion (A site), and a cation with a relatively small ion radius occupies the position of the B ion (B site). Here, considering the case of Y: Ba: Cu = 30: 43: 27 in the above report, Ba ions occupy the A site and Cu ions occupy the B site. Y ions must be distributed to the A and B sites to the same extent. However, since it is very difficult for Y ions to occupy the B site, unstable Y may be deposited in the form of Y ₂ O ₃ . In the case where the bonding characteristics are not actually good, precipitation of Y ₂ O ₃ is confirmed near the interface. In addition, in the case of a composition in which the Cu atoms in the barrier layer are extremely insufficient, Cu atoms diffuse near the interface of the barrier layer during the deposition of the upper superconducting electrode, causing superconducting deterioration near the junction interface. It is presumed that this causes a decrease in the superconducting transition temperature of the superconducting junction and a decrease in the IcRn product.

  On the other hand, the barrier layer of the Josephson junction element of the present invention is a film (deposited film) obtained by depositing the above-described material. Among the cations in the material, the Cu content is 35 atomic% to 55 atomic%, and the element RE Since the content is set in the range of 12 atomic% to 30 atomic%, it is possible to avoid excessive Y ions (element RE ions) and extreme shortage of Cu ions as in the case of the interface-modified bonding described above. . Therefore, it is presumed that the Josephson junction element of the present invention increases the IcRn product. Embodiments will be described below with reference to the drawings.

(First embodiment)
FIG. 2 is a cross-sectional view of the Josephson junction element according to the first embodiment of the present invention.

  Referring to FIG. 2, the Josephson junction element 10 according to the first embodiment has a ramp edge type structure. The Josephson junction element 10 includes a substrate 11, a lower electrode layer 12 formed on the substrate 11, an insulating film 13 covering the lower electrode layer 12, a surface of the substrate 11, one end of the lower electrode layer 12 and the insulating film 13. The barrier layer 14 that covers the slope formed on the insulating layer 13 and the surface of the insulating film 13, and the upper electrode layer 15 that covers the barrier layer 14. A superconducting junction 16 is formed by the lower electrode layer 12, the barrier layer 14, and the upper electrode layer 15.

The substrate 11 is selected from materials having a crystal structure on which the lower electrode layer 12 is epitaxially grown on the substrate 11. As the material of the substrate 11, for example, MgO, yttrium stabilized zirconia _{(YSZ), SrTiO 3, (} LaAlO 3) 0.3 - (SrAl 0.5 Ta 0.5 O 3) 0.7 (LSAT), LaAlO ₃ and SrTiO ₃ .

Although not shown, a buffer layer may be provided on the substrate 11, and in this case, a material having a crystal structure in which the buffer layer epitaxially grows the lower electrode layer 12 is selected. Examples of the buffer layer include SrTiO ₃ , CeO ₂ , BaZrO _{3 and the} like.

The lower electrode layer 12 and the upper electrode layer 15 are made of an oxide superconducting material containing (RE) ₁ (AE) ₂ Cu ₃ O _y as a main component. Here, the element RE is at least one selected from the group consisting of Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and the element AE is Ba, Sr. And at least one selected from the group consisting of Ca. (RE) ₁ (AE) ₂ Cu ₃ O _y has a perovskite crystal structure as a basic structure. Note that (RE) ₁ (AE) ₂ Cu ₃ O _y indicates that the molar ratio of the element RE, the elements AE, Cu, and O is 1: 2: 3: y. The main component is that the molar ratio of each element is most preferably 1: 2: 3: y, but the surface flatness of the lower electrode layer 12 and the upper electrode layer 15 is controlled and superconductivity is achieved. It means that a slight deviation of the composition may occur due to reasons such as improvement of the critical current value.

Suitable superconducting materials for the lower electrode layer 12 and the upper electrode layer 15 include, for example, YBaCuO-based YBa ₂ Cu ₃ Oy (YBCO), SmBa ₂ Cu ₃ O _y (SBCO), YbBa ₂ Cu ₃ O _y (YbBCO), and the like. It is.

The insulating film 13 is selected from an insulating material having a basic structure of a perovskite structure that epitaxially grows on the lower electrode layer 12 and epitaxially grows the upper electrode layer 15 on the insulating film 13. The insulating film 13 is preferably selected from, for example, SrSnO ₃ , SrTiO ₃ , PrGaO _{3 and the} like.

  The barrier layer 14 includes the element RE, the elements AE, Cu, and oxygen, and the element RE includes Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu. It is made of a material that is at least one of the group and the element AE is at least one of the group consisting of Ba, Sr, and Ca, and among the cations in the material, the Cu content is 35 atomic% to 55 atoms %, And the elemental RE content is set in a range of 12 atomic% to 30 atomic%, and the composition is different from that of the lower electrode layer 12 and the upper electrode layer 15. Since the barrier layer 14 has this composition and is a deposited film, a high-quality superconducting junction 16 is formed, and the IcRn product of the Josephson junction element 10 can be improved.

  FIG. 3 is a diagram illustrating the composition range of the barrier layer, and is a diagram illustrating a ternary composition of only the cations of RE, AE, and Cu. In FIG. 3, when a perpendicular is drawn from each point indicating the composition to each side, the length of the perpendicular is proportional to the content of the element at the apex facing the side. For example, the composition at point A has an RE element content of 12 atomic%, an AE element content of 33 atomic%, and a Cu element content of 55 atomic%. The numbers on the scale in FIG. 3 are shown as atomic percentages.

Referring to FIG. 3 together with FIG. 2, the barrier layer 14 is set such that the Cu content is 35 atomic% to 55 atomic% and the element RE content is 12 atomic% to 30 atomic% among the cations. . This composition range is within a range in which the points ABCD in the figure are connected by straight lines in this order. In the figure, the point M indicates the cation composition of the superconducting material of (RE) ₁ (AE) ₂₀ Cu ₃ O _y which is the main component of the superconducting material of the lower and upper electrode layers 12 and 15.

  The Cu content of the oxide constituting the barrier layer 14 is set in a range of 35 atomic% to 55 atomic%. However, when the Cu content is less than 35 atomic%, the IcRn product of the Josephson junction element 10 decreases. When it exceeds 55 atomic%, the Josephson junction element 10 is not preferable because it exhibits a flux flow (FF) type current-voltage characteristic. The FF type current-voltage characteristics will be described later.

The RE content of the oxide constituting the barrier layer 14 is set in the range of 12 atomic% to 30 atomic%. However, when the RE content is less than 12 atomic%, the element RE is insufficient and the perovskite structure is maintained. It becomes difficult to transition to another phase, and the junction exhibits a resistance type characteristic. When the RE content exceeds 30 atomic%, extra RE atoms are precipitated in the form of RE ₂ O ₃ or the like, leading to a decrease in bonding characteristics, for example, a decrease in IcRn product.

  Further, the composition of the barrier layer 14 is set to be different from the compositions of the lower and upper electrode layers 12 and 15. This is because a bonding interface is formed and the barrier layer 14 functions as a barrier layer. Further, the Cu and element RE contents of the barrier layer 14 are preferably different from the compositions of the lower and upper electrode layers 12 and 15 by 3 atomic% or more, respectively. This is because a stable barrier layer 14 can be formed.

  The film thickness of the barrier layer 14 is preferably set in the range of 0.5 nm to 6 nm. When the thickness of the barrier layer 14 is less than 0.5 nm, the Josephson junction element starts to exhibit FF type current-voltage characteristics. When the thickness exceeds 6 nm, the IcRn product may be reduced, for example, to 2 mV or less. Since the thickness of the barrier layer 14 is extremely thin as described above, the composition of the barrier layer 14 is analyzed by setting the electron beam diameter of the transmission electron microscope to 1 nm or less as will be described later. 14, and composition analysis is performed using energy dispersive X-ray spectroscopy (EDX).

  As described above, in the Josephson junction element 10 according to the first embodiment, since the barrier layer 14 is a deposited film of an oxide material, the Cu layer contains Cu as in the conventional interface-modified junction. The problem that the amount is extremely small can be avoided, and a high-quality superconducting junction 16 can be formed. As a result, the IcRn product of the Josephson junction element can be improved. Furthermore, variations in the critical current Ic can be suppressed. In addition, there is an effect that the critical temperature of the superconducting junction 16 becomes higher than that of the conventional interface reforming type. As a result, the cooling cost of the Josephson junction element can be reduced.

  Next, a method for forming a Josephson junction element according to the first embodiment will be described.

  4A to 4E are manufacturing process diagrams of the Josephson junction element according to the first embodiment.

  First, in the process of FIG. 4A, the lower electrode layer 12 and the insulating film 13 are sequentially formed on the substrate 11 using the above-described materials. For the lower electrode layer 12 and the insulating film 13, a vacuum vapor deposition method, a sputtering method, a pulse laser deposition (PLD) method, or a chemical vapor deposition method can be used. These methods can also be used when the upper electrode layer 15 and the buffer layer 31 of the Josephson junction element 30 shown in FIG. The substrate temperature is set to, for example, 300 ° C. to 800 ° C., although it varies depending on the material used.

  4B, a resist film 21 is formed on the insulating film 13, and the resist film 21 is patterned to open a position where a lamp edge is formed. Next, a reflow process is performed on the resist film 21 to form an inclined surface on the side end surface of the resist film 21.

  4C, the insulating film 13 and the lower electrode layer 12 are etched by Ar ion irradiation using the resist film 21 as a mask. The Ar ion irradiation is performed while rotating the substrate 11 so that the incident direction is inclined by, for example, 30 ° with respect to the surface of the substrate 11. As a result, inclined surfaces are formed on the side end surfaces of the insulating film 13 and the lower electrode layer 12, and then the resist film 21 is removed.

  4D, the barrier layer 14 is formed on the inclined surface of the lower electrode layer 12, the inclined surface and surface of the insulating film 13, and the exposed surface of the substrate 11. For depositing the barrier layer 14 using an oxide material, a vacuum evaporation method, a sputtering method, a PLD, or a chemical vapor deposition (CVD) method (for example, an organic metal (MO) CVD method) can be used.

In the case of using a vacuum evaporation method, a sputtering method, or a pulsed laser deposition (PLD) method for the deposition process of the barrier layer 14, the evaporation source or target includes the element RE, the element AE, Cu, and oxygen, and the element RE is Y , La, Nd, Pr, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and the element AE is a group consisting of Ba, Sr, and Ca. An oxide material that is at least one kind is used. Examples of such oxide target materials include (RE) ₁ (AE) ₂ Cu ₃ O _y , such as YBa ₂ Cu ₃ O _y , Yb _0.9 La _0.2 Ba _1.9 Cu ₃ O _y .

  In addition, it is preferable that the substrate temperature during deposition of the barrier layer 14 is set to, for example, 620 ° C. to 760 ° C. from the viewpoint of good epitaxial growth of the barrier layer 14. Further, an inert gas such as Ar gas or Ne gas may be used as the atmospheric gas in the film forming chamber, and oxygen gas may be mixed with these gases, or only oxygen gas may be used. Further, the pressure in the film forming chamber is set to, for example, 10 mTorr to 400 mTorr. Before forming the barrier layer 14, it is preferable to clean the surface of the structure shown in FIG. 4C, particularly the inclined surface of the lower electrode layer 12, by exposure to oxygen plasma.

Further, when deposited using PLD method barrier layer 14, for example, 620 ℃ 10mTorr~200mTorr pressure, the substrate temperature using oxygen gas laser irradiation energy ^{100mJ / cm 2 ~600mJ / cm 2} , as the atmosphere gas It is preferable to set ˜740 ° C. and the substrate-target distance to 40 mm to 100 mm. The barrier layer 14 can be formed to a composition equivalent to the composition of the target. However, the barrier layer 14 can be controlled to change the composition of the barrier layer 14 by controlling the laser irradiation energy, the substrate-target distance, or the pressure. Among the cations of the material constituting 14, the Cu content can be set in the range of 35 atomic% to 55 atomic%, and the element AE can be set in the range of 12 atomic% to 30 atomic%.

  Furthermore, in the case of the YBaCuO-based barrier layer 14, Y can be set to 12 atom% to 30 atom% in the cation of the oxide material constituting the barrier layer 14. For example, to increase the Cu content of the barrier layer 14, for example, the substrate-target distance is decreased and the pressure is increased. In order to increase the content of the element Y, for example, the laser irradiation energy is increased, and the distance between the substrate and the target is increased.

  The barrier layer 14 may be formed so as to cover the entire inclined surface of the lower electrode layer 12 to the extent that the lower electrode layer 12 and the upper electrode layer 15 are not in direct contact.

  Next, in the step of FIG. 4E, the upper electrode layer 15 is formed so as to cover the surface of the barrier layer 14. The upper electrode layer 15 is formed in the same manner as the lower electrode layer 12. As described above, the Josephson junction element according to the first embodiment having the superconducting junction 16 is formed. For example, a protective film of an Au film may be formed on the upper electrode layer 15.

  As described above, according to the formation method of the Josephson junction element according to the first embodiment, the barrier layer 14 is deposited and formed, compared with the conventional interface-modified barrier layer. The film quality of the lower and upper electrode layers 12 and 15 near the barrier layer 14 is improved, and the IcRn product is improved. In particular, even if the Cu content of the barrier layer 14 is such a high Cu content that current leakage occurs between the lower electrode layer 12 and the upper electrode layer 15 in the conventional interface reforming type, damage in the crystal is caused. Therefore, current leakage can be suppressed.

  FIG. 5 is a cross-sectional view of a Josephson junction element according to a modification of the first embodiment. In the figure, portions corresponding to the portions described above are denoted by the same reference numerals, and description thereof is omitted.

  Referring to FIG. 5, a Josephson junction element 20 according to a modification of the first embodiment is formed on a substrate 11, a ground plane layer 21 formed on the substrate 11, and a ground plane layer 21. The first insulating film 22, the lower electrode layer 12 covering the first insulating film 22 while filling the through hole 23 penetrating the ground plane layer 21 and the first insulating film 22, and the second covering the lower electrode layer 12. The barrier layer 14 covering the insulating film 13, the surface of the first insulating film 22, the inclined surface formed at one end of the lower electrode layer 12 and the second insulating film 13, and the surface of the second insulating film 13; An upper electrode layer 15 covering the upper electrode layer 15 and a protective film 24 covering the upper electrode layer 15. A superconducting junction 16 is formed by the lower electrode layer 12, the barrier layer 14, and the upper electrode layer 15.

The ground plane layer 21 may be selected from the same superconducting material as that of the lower electrode layer 12 and the upper electrode layer 15 in FIG. 2, and other known oxide superconducting materials may be used. The second insulating film 13 is selected from the same material as the insulating film 13 of FIG. The ground plane layer 21 has a function of blocking noises such as electromagnetic waves from the outside by being in a superconducting state and preventing malfunction of the superconducting junction 16.

  As described above, the Josephson junction element 20 according to the modification of the first embodiment is the same as the Josephson junction element 20 shown in FIG. 2 except that the ground plane layer 21 is provided, and the effect thereof is also the same.

  Next, an example according to the first embodiment will be described.

[Example 1-1]
In Example 1-1, a Josephson junction element having the same configuration as the Josephson junction element shown in FIG. 2 and having a barrier layer of a ramp-edge type deposited film was formed.

First, on a MgO substrate, a lower electrode layer (YBa ₂ having a film thickness of 200 nm at a substrate temperature of 740 ° C., Ar + 10 vol% O ₂ atmosphere, pressure of 80 mTorr using a YBa ₂ Cu ₃ Oy target by magnetron sputtering. Cu ₃ Oy film) was formed. Next, an insulating film having a film thickness of 250 nm was formed on the lower electrode layer using a SrSnO ₃ target by a magnetron sputtering method at a substrate temperature of 680 ° C. in an Ar + 50 vol% O ₂ atmosphere at a pressure of 50 mTorr.

Subsequently, the lower electrode layer and the insulating film were etched with Ar ions to form an inclined surface using a resist film having an opening in a region for forming a superconducting junction by photolithography as a mask. Further, after removing the resist film, the surfaces of the YBa ₂ Cu ₃ Oy film as the lower electrode layer and the SrSnO ₃ film as the insulating film were cleaned with oxygen plasma.

Next, using a Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y target by the PLD method, the laser irradiation energy is set to 450 mJ / cm ² , an oxygen atmosphere, a pressure of 50 mTorr, and a substrate-target distance of 70 mm. A barrier layer having a thickness of 1 nm covering the inclined surface of the lower electrode layer, the inclined surface of the insulating film, and the surface was formed.

Next, an upper electrode layer (Y _0.9 Ba) having a film thickness of 200 nm at a substrate temperature of 660 ° C., an oxygen atmosphere and a pressure of 200 mTorr using a Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y target on the barrier layer by magnetron sputtering. _1.9 La _0.2 Cu ₃ O _y film) was formed. Next, a gold electrode was vapor-deposited on the upper electrode layer, and further patterned to form 16 Josephson elements.

  The current-voltage characteristics of the 16 Josephson junction elements thus formed were measured at a temperature of 4.2K.

  FIG. 6 is a current-voltage characteristic diagram of the Josephson junction element, where (A) is Example 1-1 and (B) is Comparative Example 1-1. In both (A) and (B), the vertical axis represents current and 0.5 mA / scale, and the horizontal axis represents voltage and is 1.0 mV / scale.

  In Example 1-1, all of the 16 Josephson junction elements exhibited a resistance shunt type junction (RSJ) type current-voltage characteristic. In the RSJ type current-voltage characteristics, even if a current is applied between the lower electrode layer and the upper electrode layer, the current shows a zero voltage state within a predetermined range, and a superconducting current flows through the superconducting junction. When the current applied between the lower electrode layer and the upper electrode layer exceeds a predetermined value, the state transitions discontinuously to a finite voltage state. In the case of the RSJ type, a logic element can be configured by switching between these states, which is an essential characteristic for a logic circuit. FIG. 6A shows an example of current-voltage characteristics of Example 1-1.

  In Example 1-1, the IcRn product at 4.2 K is 3.0 mV to 4.0 mV, the average value of the critical current Ic is 0.8 mA, and the surplus current component ΔI (the finite voltage state shown in FIG. 6A) The current value when the curve is linearly extrapolated to voltage 0 is shown.) Was 10% to 30% of Ic.

  In addition, a sample for a transmission electron microscope in which the cross section of Example 1-1 was exposed, and the beam diameter of the electron beam was set to 1 nm using a transmission electron microscope and an EDX apparatus. A compositional analysis was performed. As a result, in the barrier layer, the atomic percentage of the cation constituting the barrier layer was 21 atomic% for Y, 33 atomic% for Ba, 5 atomic% for La, and 41 atomic% for Cu. The transmission electron microscope used was a JEM-2010F field emission transmission electron microscope (acceleration voltage 200 kV) manufactured by JEOL Ltd., and the EDX apparatus used a Nolan UTW Si (Li) semiconductor detector. The analysis region was 1 nm in diameter. In addition, these apparatuses were used for the composition analysis in the following Examples or Comparative Examples.

[Comparative Example 1-1]
In Comparative Example 1-1, a Josephson junction element having a lamp edge type interface-modified junction was manufactured using a target having the same material composition as Example 1-1 except for the barrier layer.

  First, in the same manner as in Example 1-1, a lower electrode layer and an insulating film were formed on a substrate, and inclined surfaces were formed on the lower electrode layer and the insulating film.

Next, the surfaces of the YBa ₂ Cu ₃ Oy film as the lower electrode layer and the SrSnO ₃ film as the insulating film were cleaned with oxygen plasma. Further, Ar ions having an acceleration voltage of 500 V were irradiated for 3 minutes, and a barrier layer was formed on the inclined surface of the lower electrode layer.

  Next, in the same manner as in Example 1-1, an upper electrode layer and a gold electrode were vapor-deposited, and further patterned to form 16 Josephson elements. When the current-voltage characteristics of the 16 Josephson junction elements formed in this way were measured at a temperature of 4.2 K, all of the 16 exhibited RSJ-type current-voltage characteristics. An example of this is shown in FIG.

  In the case of Comparative Example 1-1, the IcRn product at 4.2K is 2.0 mV to 2.5 mV, the average value of the critical current Ic is 0.6 mA, and the surplus current component ΔI is 10% to 30% of Ic. It was.

  Moreover, when the composition analysis of the barrier layer of Comparative Example 1-1 was performed, the atomic percentage of cations constituting the barrier layer was 19 atomic% for Y, 39 atomic% for Ba, 12 atomic% for La, and 30 for Cu. Atomic%.

  When Example 1-1 is compared with Comparative Example 1-1, it can be seen that Example 1-1 has an IcRn product larger than Comparative Example 1-1 by 30% or more. This is considered to be due to the Cu content (cation ratio) of the barrier layer being as large as 11 atomic%.

  FIG. 7 is an enlarged cross-sectional view of a superconducting junction for explaining a composition analysis method in the vicinity of the barrier layer.

  Referring to FIG. 7, the composition analysis of the superconducting junction of Comparative Example 1-1 consisting of the lower electrode layer, the barrier layer, and the upper electrode layer is performed in the cross section of the superconducting junction as described in Example 1-1. An electron beam having a beam diameter of 1 nm was irradiated so as to be orthogonal, and composition analysis was performed from the energy and intensity of characteristic X-rays generated by the irradiation. The beam position C is the center of the barrier layer in the film thickness direction, and the beam positions D and E are positions displaced from the beam position C by 1.5 nm and 3.0 nm, respectively, toward the lower electrode layer side. The beam positions B and A are positions displaced from the beam position C by 1.5 nm and 3.0 nm, respectively, toward the upper electrode layer 15 side.

  FIG. 8 is a Cu content distribution diagram of the superconducting junction of Example 1-1 and Comparative Example 1-1. In FIG. 8, the horizontal axis indicates the position from the center of the barrier layer, the "+" side indicates the lower electrode layer, the "-" side indicates the upper electrode layer, -3.0, -1.5, 0, 1.5, Each point of 3.0 nm corresponds to A to E in FIG.

Referring to FIG. 8, in Comparative Example 1-1, a region where Cu is lower than the electrode composition from the center of the barrier layer extends in a range of approximately −3.0 nm to +3.0 nm. On the other hand, in Example 1-1, the Cu content is lower than 47.5 atomic% of the electrode composition only at the position of 0 nm (barrier layer center), and the positions of ± 1.5 nm and ± 3.0 nm. In, it is substantially equivalent to the electrode composition. From this, in Example 1-1, the barrier layer was much thinner than Comparative Example 1-1, and the lower electrode layer and the upper electrode layer had almost no Cu defects near the barrier layer interface. The IcRn product is estimated to be improved. This is brought about by the fact that the barrier layer of the present invention is a deposited film.
[Example 1-2]
In Example 1-2, except for the barrier layer and the upper electrode layer, a target having the same material composition as Example 1-1 was used, and a barrier layer of a lamp edge type deposited film having the same configuration as Example 1-1 was provided. A Josephson junction element was fabricated.

The barrier layer uses a YBa ₂ Cu ₃ Oy target by the PLD method, the laser irradiation energy is set to 450 mJ / cm ² , the oxygen atmosphere, the pressure is 50 mTorr, and the substrate-target distance is 80 mm. A barrier layer having a film thickness of 1.5 nm covering the inclined surface and the inclined surface and surface of the insulating film was formed.

As the upper electrode layer, a 200 nm-thick upper electrode layer was formed on the barrier layer using a YBa ₂ Cu ₃ Oy target using a magnetron sputtering method at a substrate temperature of 660 ° C., an oxygen atmosphere, and a pressure of 200 mTorr.

  In Example 1-2, 16 Josephson elements were formed by patterning. All 16 Josephson junction elements formed in this way exhibited RSJ type current-voltage characteristics.

  In the case of Example 1-2, the IcRn product at 4.2 K is 2.5 mV to 3.5 mV, the average value of the critical current Ic is 0.6 mA, and the surplus current component ΔI is 10% to 30% of Ic. It was.

  Moreover, when the composition analysis of the barrier layer of Example 1-2 was conducted, it was the atomic percentage of the cation which comprises a barrier layer, Y was 32 atomic%, Ba was 33 atomic%, and Cu was 35 atomic%.

[Comparative Example 1-2]
In Comparative Example 1-2, a Josephson junction element having a lamp edge type interface-modified junction having the same configuration as Comparative Example 1-1 using a target having the same material composition as Example 1-1 except for the barrier layer Was made.

  First, in the same manner as in Example 1-1, a lower electrode layer and an insulating film were formed on a substrate, and inclined surfaces were formed on the lower electrode layer and the insulating film.

Next, the surfaces of the YBa ₂ Cu ₃ Oy film as the lower electrode layer and the SrSnO ₃ film as the insulating film were cleaned with oxygen plasma. Further, Ar ions having an acceleration voltage of 500 V were irradiated for 3 minutes, and a barrier layer was formed on the inclined surface of the lower electrode layer.

  Next, in the same manner as in Example 1-1, an upper electrode layer and a gold electrode were vapor-deposited and further patterned to form 10 Josephson elements. When the current-voltage characteristics of the ten Josephson junction elements thus formed were measured at a temperature of 4.2 K, eight of them exhibited flux flow (FF) type current-voltage characteristics. The FF type current-voltage characteristic is a characteristic that is not preferable for constructing a logic element because a sufficient switching characteristic cannot be obtained because the voltage changes substantially continuously from the zero voltage state to the finite voltage state. Moreover, although two showed the RSJ type | mold current-voltage characteristic, the extremely large surplus current component was shown compared with Example 1-1.

  In the case of Comparative Example 1-2, the IcRn product at 4.2K cannot be calculated because Rn cannot be derived. The average value of the critical current Ic was about 8 mA and the surplus current component was 98%.

  Moreover, when the composition analysis of the barrier layer of Comparative Example 1-2 was conducted, it was found that Y was 29 atomic%, Ba was 36 atomic%, and Cu was 35 atomic% in terms of atomic percentage of the cation constituting the barrier layer.

  As described above, when Example 1-2 and Comparative Example 1-2 are compared, the Cu content in the cation of the barrier layer is equal to 35 atomic%, but when compared with Example 1-2, Comparative Example 1- It can be seen that the element characteristics of the Josephson junction element No. 2 are very inferior. This is presumed that the barrier layer of Example 1-2 is a deposited film, and that a superconducting junction similar to that of Example 1-1 is not formed.

[Comparative Example 1-3]
In Comparative Example 1-3, a Josephson junction element having a barrier layer of a lamp edge type deposited film having the same configuration as that of Example 1-1 was manufactured using a target having the same material composition as that of Example 1-1.

For the barrier layer, the surface of the YBa ₂ Cu ₃ Oy film as the lower electrode layer and the surface of the SrSnO ₃ film as the insulating film are cleaned with oxygen plasma, and then a target of Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y is obtained by PLD method. Is set to a laser irradiation energy of 600 mJ / cm ² , an oxygen atmosphere, a pressure of 150 mTorr, and a substrate-target distance of 50 mm to cover the substrate, the inclined surface of the lower electrode layer, the inclined surface of the insulating film, and the surface. A 0.5 nm barrier layer was formed.

  In Comparative Example 1-3, 16 Josephson elements were formed by patterning. When the current-voltage characteristics of the 16 Josephson junction elements thus formed were measured at a temperature of 4.2 K, all of the 16 exhibited FF-type current-voltage characteristics.

  In the case of Comparative Example 1-3, the IcRn product at 4.2K cannot be calculated because Rn cannot be derived, and the average value of the critical current Ic was about 10 mA or more.

  Moreover, when the composition analysis of the barrier layer of Comparative Example 1-3 was performed, the barrier layer was the atomic percentage of the cation constituting the barrier layer, Y was 15 atomic%, Ba was 26 atomic%, and La was 3 atomic%. , Cu was 56 atomic%.

[Comparative Example 1-4]
In Comparative Example 1-4, a Josephson junction element having a barrier layer of a ramp edge type deposited film having the same configuration as that of Example 1-1 was manufactured using a target having the same material composition as that of Example 1-1.

For the barrier layer, the surface of the YBa ₂ Cu ₃ Oy film as the lower electrode layer and the surface of the SrSnO ₃ film as the insulating film are cleaned with oxygen plasma, and then a target of Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y is obtained by PLD method. Is used to set the laser irradiation energy to 400 mJ / cm ² , an oxygen atmosphere, a pressure of 50 mTorr, and a substrate-target distance of 90 mm to cover the substrate, the inclined surface of the lower electrode layer, the inclined surface of the insulating film, and the surface. A 3 nm barrier layer was formed.

  In Comparative Example 1-4, 16 Josephson elements were formed by patterning. When the current-voltage characteristics of the 16 Josephson junction elements formed in this way were measured at a temperature of 4.2 K, all of the 16 exhibited RSJ-type current-voltage characteristics.

  In the case of Comparative Example 1-4, the IcRn product at 4.2 K was 1.0 mV to 2.0 mV, the average value of the critical current Ic was 0.5 mA, and the surplus current component ΔI was 10% to 30% of Ic. It was.

  Moreover, when the composition analysis of the barrier layer of Comparative Example 1-4 was conducted, the barrier layer was an atomic percentage of cations constituting the barrier layer, Y being 27 atomic%, Ba being 40 atomic%, and La being 3 atomic%. , Cu was 30 atomic%.

[Comparative Example 1-5]
In Comparative Example 1-5, a Josephson junction element having a barrier layer of a ramp edge type deposited film having the same configuration as that of Example 1-1 was manufactured using a target having the same material composition as that of Example 1-1.

For the barrier layer, the surface of the YBa ₂ Cu ₃ Oy film as the lower electrode layer and the surface of the SrSnO ₃ film as the insulating film are cleaned with oxygen plasma, and then a target of Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y is obtained by PLD method. Is set to a laser irradiation energy of 500 mJ / cm ² , an oxygen atmosphere, a pressure of 100 mTorr, a substrate-target distance of 70 mm, and a film thickness covering the inclined surface of the substrate, the lower electrode layer, the inclined surface of the insulating film, and the surface. A 5 nm barrier layer was formed.

  In Comparative Example 1-5, 16 Josephson elements were formed by patterning. When the current-voltage characteristics of the 16 Josephson junction elements formed in this way were measured at a temperature of 4.2 K, all of the 16 exhibited RSJ-type current-voltage characteristics.

In Comparative Example 1-5, the IcRn product at 4.2 K was 0.4 mV to 1.0 mV, and the average critical current Ic was 0.1 mA.

  Moreover, when the composition analysis of the barrier layer of Comparative Example 1-5 was conducted, the barrier layer was an atomic percentage of the cation constituting the barrier layer, and Y was 31 atomic%, Ba was 40 atomic%, and La was 3 atomic%. , Cu was 36 atomic%.

[Comparative Example 1-6]
In Comparative Example 1-6, a Josephson junction element having a barrier layer of a ramp-edge type deposited film was manufactured using a target having the same material composition as in Example 1-1.

For the barrier layer, the surface of the YBa ₂ Cu ₃ Oy film as the lower electrode layer and the surface of the SrSnO ₃ film as the insulating film are cleaned with oxygen plasma, and then a target of Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y is obtained by PLD method. Is set to a laser irradiation energy of 400 mJ / cm ² , an oxygen atmosphere, a pressure of 400 mTorr, and a substrate-target distance of 50 mm, covering the substrate, the inclined surface of the lower electrode layer, the inclined surface of the insulating film, and the surface. A 3 nm barrier layer was formed.

  In Comparative Example 1-6, 16 Josephson elements were formed by patterning. When the current-voltage characteristics of the 16 Josephson junction elements thus formed were measured at a temperature of 4.2 K, all of the 16 exhibited FF-type current-voltage characteristics.

  Further, in the case of Comparative Example 1-6, the IcRn product at 4.2 K cannot be calculated because Rn cannot be derived, and the average value of the critical current Ic was about 7 mA and the surplus current component was 98%.

Moreover, when the composition analysis of the barrier layer of Comparative Example 1-6 was performed, the barrier layer was the atomic percentage of the cation which comprises a barrier layer, Y was 11 atomic%, Ba was 33 atomic%, La was 3 atomic%. , Cu was 53 atomic%.
[Example 2]
In Example 2, a lamp edge type deposited film was formed in substantially the same manner as Example 1-1 except that the lower electrode layer, the barrier layer, and the upper electrode layer were formed using a target of SmBa ₂ Cu ₃ Oy. A Josephson junction element having a barrier layer was formed.

The lower electrode layer was formed to a film thickness of 200 nm using a SmBa ₂ Cu ₃ Oy target by a magnetron sputtering method at a substrate temperature of 780 ° C., an Ar + 10 vol% O ₂ atmosphere, and a pressure of 100 mTorr. The upper electrode layer was formed to a thickness of 200 nm using a SmBa ₂ Cu ₃ Oy target at a substrate temperature of 760 ° C., an Ar + 10 vol% O ₂ atmosphere, and a pressure of 100 mTorr.

The barrier layer uses a SmBa ₂ Cu ₃ Oy target by the PLD method, the laser irradiation energy is set to 500 mJ / cm ² , the oxygen atmosphere, the pressure is 50 mTorr, and the substrate-target distance is 60 mm. A barrier layer having a thickness of 1 nm covering the inclined surface and the inclined surface and surface of the insulating film was formed.

  In Example 2, 16 Josephson elements were formed by patterning. All 16 Josephson junction elements formed in this way exhibited RSJ type current-voltage characteristics.

  In Example 2, the IcRn product at 4.2 K was 3.0 mV to 3.8 mV, the average value of the critical current Ic was 0.9 mA, and the surplus current component ΔI was 30% of Ic.

  Moreover, when the composition analysis of the barrier layer of Example 2 was performed, Sm was 23 atomic%, Ba was 32 atomic%, and Cu was 45 atomic% by the atomic percentage of the cation which comprises a barrier layer.

[Example 3]
In Example 3, the lower electrode layer is formed using a target of Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y , the upper electrode layer is formed using a target of Yb _0.9 Ba _1.9 La _0.2 Cu ₃ O _y , and the barrier layer is further formed of Yb _0.9. A Josephson junction having a barrier layer of a ramp-edge type deposited film in substantially the same manner as in Example 1-1, except that each film was formed using a Ba _1.9 La _0.2 Cu ₃ O _y target with different film thicknesses. An element was formed.

The lower electrode layer was formed to a thickness of 200 nm using a Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y target by magnetron sputtering and a substrate temperature of 740 ° C., an Ar + 10 vol% O ₂ atmosphere, and a pressure of 80 mTorr. The upper electrode layer was formed to a thickness of 200 nm using a Yb _0.9 Ba _1.9 La _0.2 Cu ₃ O _y target at a substrate temperature of 660 ° C., an oxygen atmosphere, and a pressure of 200 mTorr.

For the barrier layer, a target of Yb _0.9 Ba _1.9 La _0.2 Cu ₃ O _y is used by the PLD method, the laser irradiation energy is set to 450 mJ / cm ² , oxygen atmosphere, pressure 70 mTorr, and substrate-target distance 70 mm. The film thickness of the barrier layer was varied in the range of 0.5 nm to 5 nm. The film thickness of the barrier layer was determined from a photograph obtained by photographing a cross section near the joint with a transmission electron microscope.

  FIG. 9 is a graph showing the relationship between the critical current Ic and the barrier layer thickness in Example 3 at 4.2K.

  Referring to FIG. 9, the critical current Ic decreased as the barrier layer thickness increased. From this, it can be understood that the critical current Ic of the Josephson junction element can be controlled in the deposited barrier layer by changing the thickness of the barrier layer.

  Further, in the Josephson junction element having a barrier layer thickness of 1 nm in Example 3, the IcRn product at 4.2 K was 2.6 mV to 3.4 mV, and the surplus current component ΔI was 5% to 30% of Ic. .

  In addition, when the composition analysis of the barrier layer of Example 3 was performed, the atomic percentage of the cation constituting the barrier layer, the total content of Y atoms and Yb atoms was 17 atomic%, Ba was 27 atomic%, and La was 4 Atomic% and Cu were 52 atomic%.

[Comparative Example 3]
In Comparative Example 3, a Josephson junction element having a lamp edge type interface-modified junction was manufactured using a target having the same material composition as in Example 3 except for the barrier layer.

  First, in the same manner as in Example 3, a lower electrode layer and an insulating film were formed on a substrate, and inclined surfaces were further formed on the lower electrode layer and the insulating film.

Next, the surfaces of the Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y film as the lower electrode layer and the SrSnO ₃ film as the insulating film were cleaned with oxygen plasma. Further, Ar ions having an acceleration voltage of 500 V were irradiated for 3 minutes, and a barrier layer was formed on the inclined surface of the lower electrode layer.

Next, in the same manner as in Example 3, an upper electrode layer of Yb _0.9 Ba _1.9 La _0.2 Cu ₃ O _y and a gold electrode were deposited and further patterned to form a Josephson element.

  FIG. 10 is a graph showing the relationship between the IcR product and the temperature in Example 3 and Comparative Example 3. FIG. 10 shows a case where the thickness of the barrier layer of Example 3 is 1 nm.

  Referring to FIG. 10, in Example 3, the IcRn product is larger than that in Comparative Example 3 over the entire temperature range from 4.2 K to 80 K, and Example 3 has a conventional interface modification over a wide temperature range. It turns out that it is superior to the comparative example 3 of quality type | mold joining. Further, the critical temperature is higher in Example 3 than in Comparative Example 3, and the operation is possible at a higher temperature.

[Example 4]
In Example 4, a Josephson junction element having a barrier layer of a ramp-edge type deposited film having the same configuration as the Josephson junction element shown in FIG. 5 was formed. A YBa ₂ Cu ₃ Oy film was used for the ground plane layer of Example 4, and a Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y film was used for the lower electrode layer and the upper electrode layer.

First, a 200 nm thick ground plane layer is formed on a MgO substrate using a YBa ₂ Cu ₃ Oy target at a substrate temperature of 740 ° C., an Ar + 10 vol% O ₂ atmosphere, and a pressure of 80 mTorr using a magnetron sputtering method. Furthermore, a first insulating film having a thickness of 250 nm was formed using a SrSnO ₃ target.

  Next, a resist mask having an opening on the surface of the first insulating film was formed by photolithography, and through holes were formed in the first insulating film and the ground plane layer by dry etching.

Next, using a Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y target using a magnetron sputtering method, a lower electrode layer (Y _0.9 Ba) having a substrate temperature of 720 ° C., an Ar + 10 vol% O ₂ atmosphere, a pressure of 80 mTorr, and a thickness of 200 nm. _1.9 La _0.2 Cu ₃ O _y film) was formed. Next, a second insulating film having a film thickness of 250 nm was formed on the lower electrode layer by using a magnetron sputtering method and using a SrSnO ₃ target at a substrate temperature of 680 ° C., an Ar + 50 vol% O ₂ atmosphere, and a pressure of 50 mTorr. .

Next, in the same manner as in Example 1-1, the lower electrode layer and the second insulating film are etched to form an inclined surface, and after further removing the resist film, the YBa ₂ Cu ₃ Oy film that is the lower electrode layer and the _second electrode layer are removed. The surface of the SrSnO ₃ film which is the two insulating film was cleaned with oxygen plasma.

Next, by using a target of Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y by the PLD method, the laser irradiation energy is set to 450 mJ / cm ² , the oxygen atmosphere, the pressure is 50 mTorr, the substrate, the inclined surface of the lower electrode layer, A barrier layer having a thickness of 1 nm was formed to cover the inclined surface and the surface of the second insulating film.

Next, an upper electrode layer having a film thickness of 200 nm is formed on the barrier layer by using a magnetron sputtering method and using a Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y target at a substrate temperature of 660 ° C., an oxygen atmosphere, and a pressure of 200 mTorr. Y _0.9 Ba _1.9 La _0.2 Cu ₃ O _y film) was formed. Next, a gold electrode was vapor-deposited on the upper electrode layer, and further patterned to form 16 Josephson elements.

  All 16 Josephson junction elements formed in this way exhibited RSJ type current-voltage characteristics.

  In the case of Example 4, the IcRn product at 4.2 K was 3.0 mV to 3.9 mV, the average value of the critical current Ic was 0.8 mA, and the surplus current component ΔI was 10% to 30% of Ic. This is about 30% or more larger than the conventionally reported IcRn product.

  Moreover, when the composition analysis of the barrier layer of Example 4 was conducted, the atomic percentage of cations constituting the barrier layer was 22 atomic% for Y, 34 atomic% for Ba, 5 atomic% for La, and 39 atomic% for Cu. Met.

  FIG. 11 is a La element distribution diagram of the superconducting junction of Example 4. In FIG. 8, the horizontal axis indicates the position from the center of the barrier layer, the "+" side indicates the lower electrode layer, the "-" side indicates the upper electrode layer, -3.0, -1.5, 0, 1.5, Each point of 3.0 nm corresponds to A to E in FIG.

  Referring to FIG. 11, the barrier layer (in the range of ± 1.5 nm from the center of the barrier) has a La content 1.5 times that of the lower electrode layer and the upper electrode layer, and the La content in the barrier layer Has increased significantly. As a result, since La has an ion radius larger than that of Y, Ba, and Cu, a bonding interface whose composition changes sharply between the barrier layer and the lower electrode layer or the upper electrode layer is formed, and the bonding interface is stabilized. It is assumed that it contributes to In addition to La, the same phenomenon has been confirmed by Pr, Nd, Sm, and Eu, which are rare earth elements having an ionic radius larger than 0.95Å.

[Comparative Example 4]
In Comparative Example 4, a Josephson junction element having a lamp edge type interface-modified junction having the same film thickness as each layer other than the barrier layer using a target having the same material composition as in Example 4 was produced.

First, in the same manner as in the fourth embodiment, the lower electrode layer that fills the through holes of the ground plane layer, the first insulating film, the first insulating film, and the ground plane layer on the substrate and covers the surface of the first insulating film, A second insulating film covering the lower electrode layer was formed. Further, in the same manner as in Example 4, inclined surfaces were formed in the lower electrode layer and the second insulating film, and the surfaces of the SrSnO ₃ film as the lower electrode layer and the second insulating film were cleaned with oxygen plasma.

  Next, Ar ions having an acceleration voltage of 500 V were irradiated for 3 minutes to form a barrier layer on the inclined surface of the lower electrode layer.

  Next, in the same manner as in Example 4, an upper electrode layer and a gold electrode were vapor-deposited, and further patterned to form 16 Josephson elements.

  All 16 Josephson junction elements formed in this way exhibited RSJ type current-voltage characteristics.

  In the case of Comparative Example 4, the IcRn product at 4.2 K was 1.5 mV to 2.2 mV, the average value of the critical current Ic was 1.3 mA, and the surplus current component ΔI was 15% to 30% of Ic.

  Further, when the composition analysis of the barrier layer of Comparative Example 4 was performed, the atomic percentage of cations constituting the barrier layer was 27 atomic% for Y, 42 atomic% for Ba, 3 atomic% for La, and 28 atomic% for Cu. Met.

  When the Example 4 and the Comparative Example 4 are compared as described above, the critical current Ic at 4.2 K is substantially the same, but the IcRn product is 1.3 times to 1.6 times that of the Comparative Example 4 in the Example 4. A large value was shown.

(Second Embodiment)
The second embodiment of the present invention is a Josephson junction element having a stacked structure.

  FIG. 12 is a cross-sectional view of a Josephson junction element according to the second embodiment of the present invention. In the figure, portions corresponding to the portions described above are denoted by the same reference numerals, and description thereof is omitted.

  Referring to FIG. 12, a Josephson junction element 30 according to the second embodiment includes a substrate 11, a buffer layer 31 formed on the substrate 11, and a lower electrode layer 12 formed on the buffer layer 31. A barrier layer 34 covering a part of the lower electrode layer 12, an insulating film 33 covering the surface of the lower electrode layer 12 other than the surface on which the barrier layer 34 is formed, and an upper electrode covering the barrier layer 34 and the insulating film 33 It is composed of layer 15. Superconducting junction 36 is formed by lower electrode layer 12, barrier layer 34, and upper electrode layer 15. The lower electrode layer 12 and the upper electrode layer 15 preferably have a perovskite crystal structure as a basic structure, and an atomic plane made of copper and oxygen is preferably formed perpendicular to the surface of the substrate 11.

The buffer layer 31 is formed by epitaxial growth on the surface of the substrate 11. Further, the lower electrode layer 12 is epitaxially grown on the buffer layer 31, and the c-axis of the crystal axis of the lower electrode layer 12 is oriented parallel to the film surface. Selected from materials. A suitable material for the buffer layer 31 is selected from oxide materials having a perovskite structure such as PrGaO ₃ , SrTiO ₃ , and NdGaO ₃ .

The substrate 11 is preferably made of SrTiO ₃ , LSAT, LaAlO ₃ , NdGaO ₃ or the like from the viewpoint of satisfactorily epitaxially growing the buffer layer 31.

  The insulating film 33 is selected from the same material as the buffer layer 31. The insulating film 33 is formed by epitaxial growth on the surface of the lower electrode layer 12.

  The barrier layer 34 contains the same material as the barrier layer 14 of the first embodiment shown in FIG. 2, that is, the element RE, the element AE, Cu, and oxygen, and the element RE includes Y, La, Nd, Pr. , Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and the element AE is at least one of the group consisting of Ba, Sr, and Ca. Among the cations in the material, the Cu content is set in the range of 35 atomic% to 55 atomic%, the RE content is set in the range of 12 atomic% to 30 atomic%, the lower electrode layer 12 and the upper electrode The composition is different from that of the layer 15.

  According to the second embodiment, the IcRn product is improved because the barrier layer 34 is a deposited film made of the same material as the barrier layer of the first embodiment. Further, the lower electrode layer 12 and the upper electrode layer 15 have crystal axes a and b that are perpendicular to the substrate surface. In the oxide superconductor, current flows easily along the a and b axis directions. Therefore, the critical current Ic increases and the IcRn product is improved when the a and b axes are perpendicular to the bonding surface than when the current is not perpendicular. . Therefore, according to the second embodiment, a stacked Josephson junction element having an improved IcRn product is realized.

  Next, a method for forming a Josephson junction element according to the second embodiment will be described with reference to FIG.

First, for example, a buffer layer 31, a lower electrode layer 12, a barrier layer 34, and an upper electrode layer 15 are deposited in this order on a substrate 11 of SrTiO ₃ . For the buffer layer 31, the lower electrode layer 12, and the upper electrode layer 15, a vacuum deposition method, a sputtering method, a PLD method, or a chemical vapor deposition method can be used. The buffer layer 31 is formed, for example, by depositing, for example, 20 nm in thickness using, for example, a PrGaO ₃ target by sputtering. The lower electrode layer 12 is deposited, for example, by sputtering, for example, using a YBa ₂ Cu ₃ Oy target, for example, heated to a substrate temperature of 600 ° C., and in a reduced pressure atmosphere of oxygen gas 50 vol% and Ar gas 50 vol% in a pressure of 20 Pa Formed. The barrier layer 34 is formed in the same manner as in the first embodiment. For example, the barrier layer 34 is formed by depositing, for example, a film having a thickness of 3 nm using a YBa ₂ Cu ₃ Oy target by the PLD method.

  Next, a resist film (not shown) that covers the region where the superconducting junction 36 is formed is formed by photolithography, and further, the upper electrode layer 15, the barrier layer 34, and a part of the lower electrode layer 12 are formed by Ar ion irradiation. Etching treatment to remove is performed.

  Next, an insulating film 33 covering the lower electrode layer 12 and the superconducting junction 36 is formed in the same manner as the buffer layer 31, and the resist film and unnecessary insulating film 33 thereon are removed by a lift-off method.

  Next, a second upper electrode layer 15-1 is formed so as to cover the upper electrode layer 15 and the insulating film 33. The second upper electrode layer 15-1 is formed in the same manner as the lower electrode layer 12. Further, although not shown, an electrode pad is formed on the surface of the second upper electrode layer 15-1. As a result, the laminated Josephson junction element 30 is formed.

(Third embodiment)
13 is a cross-sectional view of a superconducting junction circuit according to the third embodiment of the present invention, and FIG. 14 is an equivalent circuit diagram of FIG. Note that FIG. 5 shown above is a cross-sectional view taken along line AA of FIG. In the figure, portions corresponding to the portions described above are denoted by the same reference numerals, and description thereof is omitted.

  Referring to FIGS. 13 and 14 together with FIG. 5, the superconducting junction circuit 40 according to the third embodiment is a superconducting junction line in which two Josephson junction elements 41 are connected. In the superconducting junction circuit 40, as shown in FIG. 14, the Josephson junction element 41 is connected so that the respective inductors 43 are in series, and the direct current source 42 and the superconducting junction 16 are connected at the connection point between the inductors 43. Is connected.

  The superconducting junction circuit 40 has the configuration shown in FIG. The substrate 11, the lower electrode layer 12, the barrier layer 14, and the upper electrode layer 15 are respectively the same as the substrate 11, the lower electrode layer 12, the barrier layer 14, and the upper electrode layer 15 of the Josephson junction element shown in FIG. Selected from materials. The first and second insulating films 22 and 13 are selected from the same material as the insulating film 13 of the Josephson junction element shown in FIG.

  The lower electrode layer 12, the barrier layer 14, and the upper electrode layer 15 form a lamp edge type superconducting junction 16. As a form of the Josephson junction element of the third embodiment, the stacked type of the second embodiment may be used.

  As shown in FIG. 13, the upper electrode layer 15 of the Josephson junction element 41 has a predetermined junction width, and a superconducting junction 16 is formed between the upper electrode layer 15 and the lower electrode layer 12 via the barrier layer 14. Yes. The upper electrode layer 15 connects the Josephson junction elements 41 to each other to form an inductor 43 shown in FIG. Further, a bias current is supplied to the Josephson junction element 41 via the upper electrode layer 15 by a direct current source 42.

  The lower electrode layer 12 is in contact with the ground plane layer 21 through a through hole 23 formed in a part of the first insulating film 22. Although not shown, the ground plane layer 21 is grounded.

  The basic operation of the superconducting junction circuit 40 is as follows. That is, when a current pulse accompanying magnetic flux quantum is supplied from the input side, it is superimposed on the bias current flowing through the superconducting junction 16 on the left side of the paper, and as shown in FIG. 7, from the “zero voltage state” which is the superconducting state, The current flowing through the superconducting junction 16 exceeds the critical current Ic, and switching to the “finite voltage state” occurs. Furthermore, by switching the superconducting junction 16 to the “finite voltage state”, a current pulse associated with the magnetic flux flows to the superconducting junction 16 on the right side of the page, and from the “zero voltage state” to the “finite voltage state” in the same operation. Switching occurs. In this way, the flux quanta are transmitted through the Josephson junction element 41 one after another. The superconducting junction circuit 40 can control the magnitude of the critical current Ic by the film thickness of the barrier layer 14.

  The superconducting junction circuit according to the third embodiment can be operated at high speed because it is composed of the Josephson junction element 41 having a larger IcRn product than the conventional one. Further, since the critical temperature of the superconducting junction 16 is high, it can be operated at a high temperature. As a result, the cooling cost can be reduced and the apparatus can be downsized.

  The preferred embodiment of the present invention has been described in detail above, but the present invention is not limited to the specific embodiment, and various modifications can be made within the scope of the present invention described in the claims.・ Change is possible.

It is sectional drawing of the conventional Josephson junction element. It is sectional drawing of the Josephson junction element which concerns on the 1st Embodiment of this invention. It is a figure which shows the composition range of a barrier layer. (A)-(E) is a manufacturing-process figure of the Josephson junction element which concerns on 1st Embodiment. It is sectional drawing of the Josephson junction element which concerns on the modification of 1st Embodiment. It is a current-voltage characteristic figure of a Josephson junction element, (A) is Example 1-1 and (B) is comparative example 1. It is an expanded sectional view of the junction part for demonstrating the composition analysis method of a barrier layer. It is Cu content distribution map of the junction part of Example 1-1 and Comparative Example 1-1. 6 is a relationship diagram between critical current and barrier layer thickness in Example 3. FIG. It is a relationship figure of IcR product of Example 3 and Comparative Example 3 and temperature. 6 is a La content distribution diagram of a joint portion of Example 4. FIG. It is sectional drawing of the Josephson junction element which concerns on the 2nd Embodiment of this invention. It is sectional drawing of the superconducting junction circuit which concerns on the 3rd Embodiment of this invention. FIG. 14 is an equivalent circuit diagram of FIG. 13.

Explanation of symbols

10, 30, 41 Josephson junction element 11 substrate 12 lower electrode layer 12a
13, 33 Insulating film (second insulating film)
14, 34 Barrier layer 15 Upper electrode layer 16, 36 Superconducting junction 21 Ground plane layer 22 First insulating film 23 Through hole 24 Protective film 31 Buffer layer 40 Superconducting junction circuit 42 DC current source 43 Inductor

Claims (11)

A Josephson junction element in which a first superconducting electrode layer, a barrier layer, and a second superconducting electrode layer are stacked in this order,
The first and second superconducting electrode layers are made of an oxide superconducting material whose main component is (RE) ₁ (AE) ₂ Cu ₃ O _y , and the element RE includes Y, La, Pr, Nd, Sm, And at least one selected from the group consisting of Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and the element AE is at least one selected from the group consisting of Ba, Sr, and Ca.
The barrier layer is made of a material containing the element RE, the element AE, Cu, and oxygen, and among the cations in the material, the Cu content is 35 atomic% to 55 atomic%, and the element RE content is 12 atomic%. is set in a range of 30 atomic% will be, and Ri composition as the first and second superconducting electrode layers Do different,
The composition of the barrier layer has a Cu and element RE content different from the compositions of the first and second superconducting electrode layers by 3 atomic% or more , respectively. A substrate,
On the substrate, a first superconducting electrode layer having an inclined end surface on one side;
On the first superconducting electrode layer, an insulating film having an inclined surface continuous with the inclined surface formed on an end surface;
A barrier layer deposited on the surface of the inclined surface of the first superconducting electrode layer;
A second superconducting electrode layer covering the barrier layer,
The first and second superconducting electrode layers are made of an oxide superconducting material whose main component is (RE) ₁ (AE) ₂ Cu ₃ O _y , and the element RE includes Y, La, Pr, Nd, Sm, And at least one selected from the group consisting of Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and the element AE is at least one selected from the group consisting of Ba, Sr, and Ca.
The barrier layer is made of a material containing the element RE, the element AE, Cu, and oxygen, and among the cations in the material, the Cu content is 35 atomic% to 55 atomic%, and the element RE content is 12 atomic%. is set in a range of 30 atomic% will be, and Ri composition as the first and second superconducting electrode layers Do different,
The composition of the barrier layer has a Cu and element RE content different from the compositions of the first and second superconducting electrode layers by 3 atomic% or more , respectively. A substrate,
A first superconducting electrode layer formed on the substrate;
An insulating film having a through hole along the thickness direction on the first superconducting electrode layer;
A barrier layer deposited on the surface of the first superconducting electrode layer in the through hole;
A second superconducting electrode layer filling the through hole and covering the barrier layer and the insulating film,
The first and second superconducting electrode layers are made of an oxide superconducting material whose main component is (RE) ₁ (AE) ₂ Cu ₃ O _y , and the element RE includes Y, La, Pr, Nd, Sm, And at least one selected from the group consisting of Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and the element AE is at least one selected from the group consisting of Ba, Sr, and Ca.
The barrier layer is made of a material containing the element RE, the element AE, Cu, and oxygen, and among the cations in the material, the Cu content is 35 atomic% to 55 atomic%, and the element RE content is 12 atomic%. is set in a range of 30 atomic% will be, and Ri composition as the first and second superconducting electrode layers Do different,
The composition of the barrier layer has a Cu and element RE content different from the compositions of the first and second superconducting electrode layers by 3 atomic% or more , respectively.   The first superconducting electrode layer, the insulating film, and the second superconducting electrode layer have a perovskite crystal structure as a basic crystal structure, and an atomic plane made of Cu and oxygen of the first and second superconducting electrode layers is a substrate surface The Josephson junction element according to claim 3, wherein the Josephson junction element is formed perpendicularly to the surface. The barrier layer is composed of Y as the element RE and Ba as the element AE, and among the cations in the barrier layer, the Cu content is 35 atomic% to 55 atomic%, and the Y content is 12 atomic% to 30 atomic%. wherein among the claim 1-4, Josephson device according to any one claim characterized by comprising set in a range of. The first and second superconducting electrode layers and the barrier layer contain an element LRE, and the element LRE is selected from at least one selected from the group consisting of La, Ce, Pr, Nd, Sm, and Eu,
Of claims 1-4, wherein the element LRE content of the barrier layer is greater than the LRE content of the first and second superconducting electrode layers, the Josephson device according to any one claim. A superconducting junction circuit comprising the Josephson junction element according to any one of claims 1 to 6, wherein the magnetic flux quantum is a signal carrier. Forming a first superconducting electrode layer of an oxide superconducting material on a substrate;
Forming an insulating film covering the first superconducting electrode layer;
Removing a portion of the insulating film to expose the first superconducting electrode layer;
Depositing a barrier layer on the exposed surface of the first superconducting electrode layer using an oxide material;
Forming a second superconducting electrode layer of an oxide superconducting material covering the barrier layer,
The oxide material is made of a material containing the element RE, the element AE, Cu, and oxygen, and among the cations in the material, the Cu content is 35 atomic% to 55 atomic%, and the element RE content is 12 atoms. % To 30 atomic%, and the composition is different from that of the first and second superconducting electrode layers, and the element RE is Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho. , Er, Tm, is at least one of the group consisting of Yb, and Lu, Ri least 1 Tanedea of the group said element AE is composed of Ba, Sr, and Ca,
The method of forming a Josephson junction element, wherein the composition of the barrier layer is such that the Cu and element RE contents differ from the compositions of the first and second superconducting electrode layers by 3 atomic% or more, respectively . The first and second superconducting electrode layers are made of an oxide superconducting material whose main component is (RE) ₁ (AE) ₂ Cu ₃ O _y , and the element RE includes Y, La, Pr, Nd, Sm, It is at least one selected from the group consisting of Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and the element AE is at least one selected from the group consisting of Ba, Sr, and Ca A method for forming a Josephson junction device according to claim 8 . 10. The method for forming a Josephson junction element according to claim 8, wherein the step of depositing the barrier layer uses any one of a vacuum vapor deposition method, a sputtering method, a pulse laser deposition method, and a chemical vapor deposition method. . The step of depositing the barrier layer uses vacuum deposition, sputtering, or pulsed laser deposition to control the composition of the barrier layer formed based on the pressure and the distance between the substrate and the target in an oxygen gas atmosphere. 10. A method for forming a Josephson junction element according to claim 8 or 9 .

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